There is a disclosure known as a conventional propulsion unit, for example, in Japanese Patent National Publication No. 2013-519574 (PTD 1).
The propulsion unit disclosed in PTD 1 serves as an outboard motor attached to the outside of a ship at the rear end of a ship body. The propulsion unit has a configuration in which a horizontal crankshaft engine is mounted such that a crankshaft is disposed horizontally, and the driving force from the engine is transmitted to a propeller shaft using the first transmission, the second transmission and the third transmission. The crankshaft of the horizontal crankshaft engine extends rearward from the ship. Thus, the first transmission is disposed further rearward of the rear end of the ship body with the engine interposed therebetween.
Furthermore, the propulsion unit includes, on its attachment portion to the ship body, a steering shaft extending in the up-down direction. The propulsion unit is caused to swing around the axis of this steering shaft, thereby allowing the steering operation of the ship.